Erosion caused by airborne particles is a severe problem in the operation of a turbomachine in an environment of airborne abrasive particles, such as, ash, dust or sand. Erosion generally occurs on airfoil surfaces of turbomachine blades, impellers or vanes which are subjected to impacts of incoming ash, dust and/or sand. The 90.degree. erosion damage, i.e., damage resulting from sand, dust or ash particles striking the airfoil surface head-on, e.g., or at a 90.degree. angle, occurs primarily at the leading edge of the airfoil surface, while the low angle erosion damage, i.e., damage resulting from sand, dust or ash particles striking the airfoil surface at an angle less than 90.degree., e.g., at a 20.degree. or 30.degree. angle, appears primarily on the airfoil pressure surface and t railing edge. The former leads to a decrease in the cord width and/or geometrical distortion of the leading edge of the airfoil surface; and the latter results in a pitted and roughened pressure surface of the airfoil and/or loss of cord width due to eroding away of the airfoil trailing edge, which is generally thin. As a result of erosion, the aerodynamic performance of the airfoil is drastically reduced and the useful airfoil life is shortened.
Heretofore, fan, compressor or turbine airfoils have been coated or made with a wide variety of erosion-resistance materials to improve their resistance to erosion caused by abrasive particles, such as, ash, dust and/or sand. U.S. Pat. No. 4,418,124 discloses the manufacture of gas turbine engine, spray cast, superalloy airfoils, having a grain size of 0.2 to 0.5 micron, by low pressure high velocity plasma spray-casting in which fine particles of superalloy at just above its melting point are formed at high velocity in a plasma stream onto a substrate in a neutral atmosphere, low pressure chamber. Subsequent heat treatment is necessary and results in grain growth, e.g., to 2 to 3 microns. The resulting airfoils were not tested for erosion resistance which could be expected to approximate the relatively low erosion resistance known for the superalloys used.
U.S. Pat. No. 4,318,672 describes the erosion problem to which fan and turbine blades are exposed and the difficulty of meeting the problem with a single material. This patent teaches the use of two wear-resisting layers as attachments to the blades. One wear-resisting layer is made of a relatively ductile material which is relatively resistant to abrasive particles at impact angles of 45.degree. to 90.degree. but has a maximum erosion sensitivity at impact angles of 15.degree. to 30.degree.. The other wear-resistant material is a relatively hard, brittle material which is relatively resistant to erosion by particles impacting at 0.degree. to 45.degree. but has a maximum erosion sensitivity at impact angles ranging from 75.degree. to 90.degree.. The two wear-resisting layers are superimposed and detachably secured in a cut-out around the leading edge of the blade.
In U.S. Pat. No. 3,699,623, a titanium skin is diffusion bonded to the exterior surface of turbine blades made of aluminum, magnesium or alloys thereof in order to improve erosion resistance.
U.S. Pat. No. 4,492,522 discloses fan or turbine blades made of a ceramic material such as silicon carbide or silicon nitride coated with a layer of TiN, TiC, B.sub.4 C, BN or titanium carbide nitride applied by chemical vapor deposition (CVD) or physical vapor deposition (PVD). This patent fails to recognize the effect of impact angle on erosion resistance and the coatings taught by this patent may be in some cases more erodible than the ceramic substrate, especially at low impact angles.
A paper included in the Proceedings of the 6th International Conference on Erosion by Liquid and Solid Impact, entitled Erosion Of Corrosion Resistant Coatings For Jet Engine Compressors, H. J. Kolkman, National Aerospace Laboratory NLR of Amsterdam, The Netherlands, reported that soft coatings such as Sermetel 735 (aluminum pigmented basecoat sealed with a topcoat) having an Er(90.degree.)/Er(20.degree.) ratio of less than one on AISI 410 stainless steel substrates provides inadequate or no erosion protection for the substrate.
A Duckworth et al article in Thin Solid Films, 63, 1979, pp. 289-297, discloses ZrN coatings deposited by high rate r.f. sputtering of small rod targets at about 1000.degree. C. There is no disclosure or suggestion, however, of a ZrN coated substrate which, in use, has a surface subject to high angle impingement and a surface subject to low angle impingement, such as a turbomachine airfoil. There also is no disclosure or suggestion of a ZrN coating having a controlled grain size of 3,000 Angstroms or less, and the high temperature (1000.degree. C.) used by Duckworth in the deposition of ZrN coatings would in fact encourage the growth of ZrN grains greater than 3000 Angstroms in the coating.
W. D. Sproul in articles published in Thin Solid Films, 107, 1983, pp. 141-147, and 118, 1984, pp. 279-284, discloses very thin (i.e., less than about 5 .mu.m) zirconium nitride coating of tools using a very high rate of reactive sputtering. In a paper submitted to the 1984 AVS National Symposium in Reno, Nev., Sproul disclosed the coating of tools with very thin zirconium nitride by reactive sputtering. K. Salmenoja described in Vacuum, 36, 1-3, 1986, pp. 33-35, zirconium nitride coatings prepared by triode ion plating. None of these references, however, disclose or suggest the coating of thicker (i.e., greater than about 5 .mu.m) zirconium nitride onto a substrate which, in use, has a surface subject to high angle impingement and a surface subject to low angle impingement, such as a turbomachine engine airfoil. There also is no disclosure or suggestion of a zirconium nitride coating having a ratio of erosion rate at an impingement angle of 90.degree. to the erosion rate at an impingement angle of 20.degree. of not greater than 1.5 or having an erosion rate at all impingement angles from 20.degree. to 90.degree. at least two times less than that of the substrate at the same impingement angles.
Hard coatings have been used, heretofore, in efforts to prolong the service life of fan, compressor, or turbine airfoils. However, hard (i.e. brittle) materials, including many carbides, borides, nitrides, oxides and cermets, exhibit the characteristic of substantially lower resistance to 90.degree. erosion when compared with that at low angle erosion, e.g., less than 30.degree.. Typically the erosion rate (Er) of hard and/or brittle materials is a function of the impingement angle of the particle. Therefore, for such a hard coating the ratio of erosion rate at 90.degree. to 20.degree., Er(90.degree.)/Er(20.degree.), is about 8.5. In such cases, even though the coating is capable of providing significant protection for the fan, compressor or turbine airfoils at low angle erosion, the overall service life of the coated airfoil is determined by the lack of resistance of the coating to 90.degree. erosion.
Detonation gun coatings have been used as protective coatings for compressor airfoils in gas turbine engines. Severe leading edge erosion damage was observed on such coated airfoils after operation in a dust environment. The erosion resistance of these coatings at 20.degree. impingement angle was approximately two times the bare airfoil (Ti-6Al-4V or Inconel-718) but their erosion resistance values at 90.degree. impingement were only about one-half that of the bare airfoil. Clearly, these coatings offer very little protection, or no protection, for those surfaces of compressor airfoils subjected to high angle erosion.
Recently, titanium nitride coatings produced by physical vapor deposition (PVD) arc evaporation process were found to exhibit excellent erosion resistance at both high and low impingement angles and their Er(90.degree. )/Er(20.degree. ) value were found to be 2.6 to 3.2. However, the leading edge of titanium nitride coated compressor airfoils suffer substantially higher erosion due to 90.degree. particle impact than do the pressure surfaces of the airfoils even though the total weight loss of the coated airfoil due to erosion may be at least 10 times less than that of an uncoated airfoil.
Accordingly, the present invention provides coated articles having, in use, a surface subject to high angle impingement and a surface subject to low angle impingement, and having a relatively thick zirconium nitride coating having an average grain size of not greater than 3000 Angstroms applied by physical vapor deposition and characterized by having a thickness of at least about 5 .mu.m and having an erosion rate at all impingement angles from 20.degree. to 90.degree. at least about two times less than that of the substrate at the same impingement angles.